In the European Patent No. 0 017 958, a preparation for the abhesive coating of baking and cooking implements is described. This preparation comprises predominantly heat-curable polysiloxane resins and linear siloxanes, which can react with these polysiloxane resins. This mixture of heat-curable polysiloxane resin and reactive, linear polysiloxane is advantageously stoved or baked on the surface of the metal in the presence of conventional curing catalysts at about 200.degree. to 300.degree. C.
As heat-curable siloxane resin, a polymer can be used, which is obtained by reacting siloxanes of the general formula ##STR6## wherein OR.sup.5' is an alkoxy group of primary or secondary aliphatic alcohols with 1 to 4 carbon atoms,
with low molecular weight, multihydric alcohols at temperatures of 100.degree. to 160.degree. C., if necessary, in the presence of known transesterification catalysts and solvents with removal of the R.sup.5' OH alcohol, in such a quantitative ratio, that one SiOR.sup.5' group approximately corresponds to one COH group, with the proviso that the reaction is terminated after a 25 to 80% conversion by cooling to a temperature below 100.degree. C.
As reactive, linear polysiloxane, one having the formula ##STR7## is used, wherein R.sup.1', R.sup.2' are the same or different and represent methyl or phenyl, with the proviso that at least 80 mole percent of these groups are methyl, the R.sup.3' groups are the same or different and represent one or several moieties from the group of
The reactive linear polysiloxane is contained in the preparation in an amount of 0.05 to 4% by weight, based on the siloxane resin that has been modified with the low molecular weight, multihydric alcohol.
The surface coatings, obtained with the preparations of the European Patent No. 0 017 958, exhibit the desired abhesivity. They can be stoved already at relatively low temperatures of 200.degree. C. Their thermoplasticity is slight compared with that of the resins, which were known at that time from the art. These preparations are very suitable especially for coating baking molds, which usually are not stressed very much mechanically. However, pans and pots, which are used for roasting, grilling, broiling, frying and cooking, are subjected to substantially higher mechanical stresses. Moreover, due to the hot fat or oil, higher temperatures are frequently reached during roasting, grilling, broiling and frying than during baking.
The present invention is therefore based on the task of finding preparations, which form abhesive surface coatings on the metal surface after stoving, the hardness of which is higher than that achievable with preparations of the art. The higher hardness is to be retained especially at temperatures of 200.degree. C. and above. This so-called hot hardness is of special importance especially for the utility of the coated objects. The hot hardness must not diminish even after the frequent temperature changes, to which the coated objects are exposed. The coatings should have a high mechanical strength with a good bond to the metallic substrate. inadequate mechanical strength is a known disadvantage of the objects coated with polytetrafluoroethylene. The coatings, moreover, should not develop any physiologically hazardous degradation products if the pots and pans are inadvertently overheated.
From the published British Patent Application No. 2,152,946, abhesive coatings are known, which are obtained by stoving a silicone polyester resin. The essence of this invention is the addition of 20 to 200 parts by weight of a laminar solid, preferably mica, to 100 parts by weight of the curable silicone polyester resin. Surely certain physical properties of the cured surface layer are improved; for example, the hardness of this layer is increased and the permeability is reduced. However, these improvements are limited by the basic properties of the resin. It is therefore better to attempt to achieve the improvement in the properties of the cured surface layer by an improvement in the coating resin itself.